1. Field of the Invention
The present invention relates to a CMOS image sensor and a method for fabricating the same.
2. Discussion of the Related Art
FIG. 1 illustrates a cross sectional view of a CMOS image sensor according to the related art. FIG. 2 illustrates a cross sectional view of an active region in a CMOS image sensor according to the related art.
As shown in FIG. 1 and FIG. 2, a p-type well 2 is formed in a p-type substrate 1. Also, shallow trench isolation STI regions 3 are formed in a surface of the p-type well 2 for electric isolation of pixels.
Then, an N-type photodiode 5 is formed inside the P-type well 2 in each of cell regions 4. Also, impurity regions 6 are formed in the surface of the P-type well 2 adjacent to the N-type photodiode 5, wherein the impurity regions 6 serve as source and drain regions.
Subsequently, a poly-silicon gate electrode 7 is formed on the surface of the P-type well 2 in correspondence with the impurity regions 6. In the drawings, a voltage source Vc applies a voltage to the impurity regions 6, and a gate insulating layer 8 is provided.
As designs become smaller when forming the poly-silicon gate electrode for a CMOS transistor according to the related art, shallow trench isolation (STI) is often used to isolate the pixels from one another.
STI, trenches are formed in the surface of the P-type well 2, and then oxide layers are grown or deposited into the trenches. After that, an etch-back process is performed to the oxide layers, whereby isolation layers are formed in the trenches for isolation of the pixels.
However, when etching the surface of the flat silicon substrate 1, crumpled portions may be generated. In this case, mechanical and electrical stresses may be generated in a tilted or crumpled surface between the oxide layer inside the trench and the silicon substrate 1, thereby causing an increase in defect density. Accordingly, a leakage current occurs in the CMOS image sensor, and a dark defect occurs in images.